Magnetoresistance effect elements having a three-layer structure of ferromagnetic layer/non-magnetic layer/ferromagnetic layer undergo a change in resistance depending upon the relative direction of magnetization in two ferromagnetic layers, and, thus, can be used as magnetic memory cells that record information as a magnetization direction by associating two states, that is, particularly a state where the magnetization direction of one of the ferromagnetic layers is nearly parallel to that in the other ferromagnetic layer and a state where the magnetization direction of one of the ferromagnetic layers is nearly antiparallel to that in the other ferromagnetic layer, with “0” or “1”, respectively. Information is read by reading element resistance. The non-magnetic layer may be either a metal or an insulator. Conventionally, there has been a plurality of rewriting methods (magnetization reversal methods) in magnetoresistance effect elements. Among them, electric writing methods in which a voltage is applied across the ferromagnetic layers are advantageous from the viewpoints of higher integration and lower power consumption. Representative writing methods include spin injection magnetization reversal methods and electric field induced magnetization reversal methods.
As illustrated in FIG. 11, in the spin injection magnetization reversal method, current is applied across a three-layer structure of ferromagnetic layer/non-magnetic layer/ferromagnetic layer to perform magnetization reversal through giving and receiving of angular momentum. The direction of magnetization reversal can be regulated by the direction of current. In one example illustrated in FIG. 11, the lower electrode and the upper electrode are provided as a stationary layer that is less likely to undergo magnetization reversal than the upper electrode and a recording layer, respectively, and, when positive voltage is applied to a terminal B on the recording layer side relative to a terminal A on the stationary layer side, current flows from the recording layer side to the stationary layer side and, as a result, magnetization reversal occurs so that the magnetization direction in the recording layer is parallel to the magnetization direction of the stationary layer. When a negative voltage is applied to the terminal B relative to the terminal A, magnetization reversal occurs so that the magnetization direction of the recording layer is antiparallel to that in the stationary layer.
Thus, in the spin injection magnetization reversal method, the direction of the magnetization reversal is determined by the direction of the current, and the flow of a larger current for a longer period of time contributes to an improvement in the probability of success in information writing. For this reason, in the information writing, the stability of information writing can be ensured by applying current for a longer period of time than a magnetization reversal time expected as a necessary minimum time. The magnetization reversal time in the spin injection magnetization reversal method can be reduced by increasing the amount of current. However, it is known that, in a region where the magnetization reversal time is about 1 ns (nanosecond), the current necessary for writing is sharply increased (see, for example, Non-Patent Literature 1 or 2). Since the current applicable to the element has an upper limit, the actual writing time is approximately several ns to several tens of ns. The power consumption necessary for rewriting one memory cell once is determined by an energy loss derived from Joule heat and is about 1 pj for high-performance magnetoresistance memory cells.
In order to reduce the power consumption in the spin injection magnetization reversal method, a method has been proposed in which, after the application of a first current lower than a threshold current that is a current necessary for magnetization reversal with a single current pulse, a second current that is higher than the first current and is smaller than the threshold current value is applied (for example, see Patent Literature 1).
As illustrated in FIG. 12, the electric field induced magnetization reversal method uses a capacitor structure element having a three-layer structure of ferromagnetic layer/insulating layer/metallic layer. In one example illustrated in FIG. 12, the application of a voltage across the terminal A on the metallic layer side and the terminal B on the ferromagnetic layer side causes a change in magnetic anisotropy of the ferromagnetic layer to produce a magnetic field in a direction indicated by an arrow. In the following explanation, the direction of magnetization easy axis of the ferromagnetic layer is a perpendicular direction, and a direction parallel to the film surface is easy to magnetize by the application of a voltage. Alternatively, an arrangement may also be possible in which a magnetic field in a direction perpendicular to the film surface is produced by applying a voltage to magnetization having an in-plane magnetization easy axis (see, for example, Non-Patent Literature 3). In the magnetization of the ferromagnetic layer, the magnetization reversal can be carried out by performing precession with a produced magnetic field as an axis and cutting off the voltage when approximately half-cycle precession has been performed, to again bring the magnetization to perpendicular easy magnetization (see, for example, Non-Patent Literature 4).
The magnetization reversal time in the electric field induced magnetization reversal method is on the order of several hundreds of ps (picoseconds) to 1 ns. The polarity of the applied voltage is identical in both magnetization reversal from a parallel state to an antiparallel state and magnetization reversal from an antiparallel state to a parallel state. In writing, the voltage amplitude and the applied time (voltage pulse length) need to be regulated, and the time error acceptable when the applied voltage amplitude is fixed is on the order of several tens of picoseconds. When the element resistance through the insulating layer is sufficiently large and the energy loss by Joule heat is small, the electric power consumed in writing into one memory cell once is an electric power necessary for storing electric charges between the ferromagnetic layer and the metallic layer and is about 1 fj.